1. Field of the Invention
This invention relates to improvements in drive transmissions and, more particularly, to a continuously and automatically variable transmission for use with work producing mechanisms, such as a bicycle.
2. Description of the Prior Art
Continuously variable speed transmissions for bicycles have been patented, but have not achieved commercial application. In such a transmission, a change in the drive ratio is effected through a radially extensible or contractible sprocket assembly mounted on or driven by the bicycle pedal assembly. A disclosure covering such a variable speed transmission is U.S. Pat. No. 3,995,508.
In such patent, a plurality of small sprockets are slidably mounted in radial slots in a main disk, the sprockets being biased toward radially outermost positions and engaging a roller chain for transmitting drive torque at a maximum drive ratio. The radial positions of the sprockets are controlled by spiral grooves in a control disk that is angularly displaceable relative to the main disk against the bias of an adjustable spring which urges the sprockets to their radially outermost positions.
Variable speed transmissions of the types disclosed in the above patent and elsewhere have not been satisfactory because they are complex in construction and are not efficient because of the considerable amount of friction caused by the sliding engagement of the various parts of the transmissions. Some of the deficiencies in the other continuously variable transmissions are:
(1) Roller clutches prevent reverse rotation of sprockets, thus causing the chain to jump off the sprockets when the mechanism moves from its maximum to its minimum diameter. PA1 (2) Roller clutches are too small to survive for a long period of time under the loads imposed. PA1 (3) Metallic springs cannot produce the needed stiffness and total deflection required within the space allowed. PA1 (4) No provisions for allowing each individual sprocket to rotate forward or backward as required during chain engagement to assure that such engagement is smooth.
Because of these drawbacks, a need exists for an improved variable speed transmission which minimizes contact between parts but which provides for a continuously and automatically variable drive ratio between maximum and minimum values. The present invention satisfies this need.